1. Field of the Invention
The present invention relates to a reading lens system for use in a scanner, which is used to read an original image in a device such as a facsimile transmitter/receiver, and a digital copying machine.
2. Description of the Related Art
In connection with the facsimile transmitter/receiver or digital copying machine, it has been heretofore well known that an original image is scanned and read by means of a solid state image reading device such as a CCD (charge-coupled device). The reading lens system is used for such a purpose to form a scanned image on a solid state image reading device.
As the pixel size in the solid state image reading device has become smaller, the reading lens systems have been required to have a high resolution. For example, when the original image is read by a CCD whose pixel size is 7 .mu.m, the lens system should have a resolution of 71.4 lines/mm on a light receiving surface of the CCD, and is required to produce an image with sufficiently high contrast for the above spacial frequency from the center to the peripheral area of the imaging surface.
Image reading devices have been always required to be more compact and to perform speedier image reading. To meet such demands, the reading lens system should have a smaller distance between the object and the image surface, a high resolution, a wide field of view, and a small F number.
Examples of such reading lens systems are disclosed in Japanese Laid-open Patent Publication Nos. 075,721/1988 and 023,215/1989, wherein lens system have 5 lens units comprising 7 lenses.
Those lens system have F number (F/No) of about 4.0, which is not enough to perform high speed image reading.